The present invention relates to a light modulation apparatus for modulating the quantity of incident light and outputting the modulated light and an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and the image pickup apparatus.
Light modulation apparatuses have been known of a type including a liquid crustal cell, typically, a twisted nematic (TN) type liquid crystal cell or a guest host type liquid crystal cell (GH cell), and a polarizing plate.
FIGS. 1A and 1B are schematic views showing an operational principal of a related art light modulation apparatus mainly including a polarizing plate 1 and a GH cell 2, and FIG. 1C is a graph showing a rectangular waveform of a drive voltage to be applied to the GH cell 2. In the figures, for ease of description, components of a liquid crystal device other than the GH cell 2, for example, two glass substrates between which the GH cell 2 is held, operational electrodes, and liquid crystal alignment films formed on the substrates are omitted. The GH cell 2 contains liquid crystal molecules 3 and dichroic dye molecules 4. The dichroic dye molecules 4 have a positive type (p-type) light absorption anisotropy capable of absorbing light in the alignment direction of major axes of the molecules, and the liquid crystal molecules 3 have a positive type (p-type) dielectric constant anisotropy.
FIG. 1A shows a state of the GH cell 2 when no voltage is applied thereto. Incident light 5, which passes through the polarizing plate 1, is linearly polarized by the polarizing plate 1. In this related art light modulation apparatus, since the polarization direction of the linearly polarized light corresponds to the alignment direction of the major axes of the dichroic dye molecules 4, the light is absorbed in the dichroic dye molecules 4, with a result that the transmittance of the GH cell 2 is reduced.
When a voltage having a rectangular waveform shown in FIG. 1C is applied to the GH cell 2 as shown in FIG. 1B, the alignment direction of the major axes of the dichroic dye molecules 4 becomes perpendicular to the polarization direction of the linearly polarized light, with a result that the light is little absorbed in the GH cell 2, that is, most of the light passes through the GH cell 2.
In the case of using a GH cell including a negative type (n-type) dichroic dye molecules capable of absorbing light in the alignment direction of minor axes of the molecules, the relationship between light absorption and light transmission of the GH cell is reversed to that of the GH cell 2 including the positive type dichroic dye molecules 4. To be more specific, the light is not absorbed in the GH cell including the negative type dichroic dye molecules when no voltage is applied thereto, and light is absorbed in the GH cell including the negative type dichroic dye molecules when a voltage is applied thereto.
An optical density (absorbance) ratio of the light modulation apparatus shown in FIGS. 1A to 1C, that is, a ratio of an optical density of the apparatus upon application of a voltage to an optical density thereof upon application of no voltage is about 10. This optical density ratio of the apparatus shown in the figures is as large as about twice an optical density ratio of a light modulation apparatus including only the GH cell 2 without use of the polarizing plate 1.
The related art light modulation apparatus shown in the figures has a problem. Since the polarizing plate 1 is fixed in an effective optical path of light, part of light, for example, 50% of light is usually absorbed in the polarizing plate 1, and further light may be reflected from the surface of the polarizing plate 1. As a result, the maximum transmittance of light passing through the polarizing plate 1 cannot exceed a certain value, for example, 50%, and accordingly, the quantity of light passing through the light modulation apparatus is significantly reduced by light absorption of the polarizing plate 1. This problem is one of factors which make it difficult to put a light modulation apparatus using a liquid crystal cell into practical use.
On the other hand, various kinds of light modulation apparatuses using no polarizing plate have been proposed. Examples of these apparatuses include a type using a stack of two GH cells in which the GH cell at the first layer absorbs a polarization component in the direction identical to that of polarized light and the GH cell at the second layer absorbs a polarization component in the direction perpendicular to the polarized light; a type making use of a phase transition between a cholesteric phase and a nematic phase of a liquid crystal cell; and a high polymer scattering type making use of scattering of liquid crystal.
These light modulation apparatuses using no polarizing plate have a problem. Since the optical density (absorbance) ratio between upon application of no voltage and upon application of a voltage is, as described above, as small as only 5, the contrast ratio of the apparatus is to small to normally carry out modulation of light at any location in a wide range from a bright location to a dark location. The light modulation apparatus of the high polymer scattering type has another problem in significantly degrading, when the apparatus is used for an image pickup apparatus, the image formation performance of an optical system of the image pickup apparatus.
The related art light modulation apparatus presents a further problem. Since the transmittance in a transparent state may become dark depending on the kind of a liquid crystal device used for the apparatus, if an image pickup apparatus provided with the light modulation apparatus is intended to pickup image with a sufficient light quantity in such a transparent state, the light modulation apparatus is required to be removed from an optical system of the image pickup apparatus.
The related art light modulation apparatus has the following problem associated with the drive thereof. To drive the related art light modulation apparatus, the transmittance has been controlled by modulating a magnitude of a DC voltage or AC voltage applied to the apparatus; however, for the light modulation apparatus at a consumer level, it is difficult to accurately perform voltage control and to obtain a characteristic having a low threshold value; a limitation lies in the number of gradation of the transmittance level; and D/A conversion is required for voltage control based on the intensity of transmission light, to raise a circuit cost.
The drive of the related art light modulation apparatus, particularly, of a type including a negative type liquid crystal having a negative dielectric constant anisotropy has another problem. In the related art light modulation apparatus, the transmittance has been changed with a large step from a current transmittance into a target transmittance; however, upon such a change in transmittance with a large step, particularly, from a transmittance in a transparent state into a transmittance in a light shield state, there occurs a defect in alignment of liquid crystal molecules, resulting in unstable optical characteristics, for example, in-plane non-uniformity in transmittance (which will be described later).
To be more specific, when a voltage applied to the liquid crystal is changed with a large step for changing the transmittance with a large step, there occurs a transient state in which liquid crystal molecules are aligned in different directions, and if such a transient state continues for a time being long enough to exert an effect on the transmittance, there appears in-plane non-uniformity in transmittance. In general, the transient state disappears after an elapse of a certain time required for re-alignment of liquid crystal molecules and pigment molecules; however, in the worst case, the transient state may partially remain even after an elapse of a long time.
A further problem of the drive of the related art light modulation apparatus is that even in a state in which drive pulses with a specific control waveform are applied to a liquid crystal device of the light modulation apparatus, there occurs a variation in transmittance due to a change in temperature of the environment in which the apparatus is disposed.